Electrographic toner development employing a clean-up electrode structure for removing unwanted background



l. BRODIE 3,411,482 ELECTROGRAPHIC TONER DEVELOPMENT EMPLOYING ACLEAN-UP Nov. 19, 1968 ELECTRODE STRUCTURE FOR REMOVING UNWANTEDBACKGROUND Filed Jan. 30. 1 67 FIG. I

PRIOR ART FIG. 2 PRIOR ART 4% fit L INVENTOR. IVOR BRODIE ATTORNEYUnited States Patent M ELECTROGRAPHIC TONER DEVELOPMENT EMPLOYING ACLEAN-UP ELECTRODE STRUCTURE FOR REMOVING UNWANTED BACKGROUND IvorBrodie, Palo Alto, Calif., assignor to Varian Associates, Palo Alto,Calif., a corporation of California Filed Jan. 30, 1967, Ser. No.612,531 4 Claims. (Cl. 118-637) ABSTRACT OF THE DISCLOSUREElectrographic toner developers or inkers are disclosed employing arelatively open clean-up electrode structure in the form of one or moreslat-shaped electrodes or a metallic screen disposed over the chargeimage bearing surface of the recording medium being developed suchmedium being, for example, a strip of electrographic paper. A secondelectrode, in the form of a plate or the like, is disposed opposite theclean-up electrode structure on the other side of the recording medium.An electrical potential is applied to the clean-up electrodes toestablish an electric field adjacent the image bearing side of therecording medium to pull charged toner particles from the inked surfaceof the recording medium which are not electrostatically bound to thecharge image to be developed, thereby reducing the background.

Description of the prior art Heretofore, channel type toner developersor inkers have been employed to apply charged toner particles to chargeimages to be developed. Typical of such inkers is a channel memberextending across the charge image bear-ing surface of an electrographicpaper to be developed. The channel has a slot in one side adjacent thepaper which permits the toner particles, suspended in a stream ofinsulative air or liquid, to come into fluid communication with thecharge image to be developed. The charged toner particles areelectrostatically attracted out of suspension and electrostaticallybound to the charge image pattern thereby developing same. Increasingthe concentration of toner particles in the suspension from a fewpercent by weight to higher concentrations permits faster developmentspeeds. For example, for paper pulling speeds of 5 to inches per secondemploying a liquid toner, the toner particle concentration is preferablyincreased to about to by weight of the liquid toner. However, with suchhigh concentrations of toner particles, it is found that substantialamounts of the toner particles are left on the paper in the regionswhere they are not electrostatically bound to the charge image to bedeveloped. Therefore, they produce an unwanted background fog on thedeveloped image.

Therefore, development electrodes, in the form of one or more platesdisposed with their flat sides facing the image bearing surface beingdeveloped, have been employed to control the rate at which tonerparticles are deposited upon the charge image being developed. Suchelectrode structures are relatively closed in that the toner particlesdo not readily pass through the structure. Toner is supplied by a streamdirected between the electrode structure and the image being developed.In a high speed inker or developer, such a closed electrode structureinterferes with an adequate supply of the toner particles to the image,thereby leaving the image underdeveloped. Such development electrodestructures are described in a text entitled, Electrophotography,published in 1965 by The Focal Press of London and New York at pages32-34 and 152.

3,411,482 Patented Nov. 19, 1968 Summary of the present invention Theprincipal object of the present invention is the provision of means forreducing or eliminating background fog on toner developed electrostaticimages.

One feature of the present invention is the provision in anelectrographic toner developer or inker of a relatively open clean-upelectrode structure for producing an electric field over the imagebearing surface of the recording medium for pulling charged tonerparticles off the background image areas where they are notelectrostatically bound to the charge image being developed, wherebyunwanted background fog is removed from the developed image.

Another feature of the present invention is the same as the precedingfeature wherein the clean-up electrode structure includes an elongatedslat-shaped electrode directed across the recording medium with its thinedge facing the image bearing surface of the recording medium, wherebyan intense clean-up electric field is produced adjacent the imagebearing surface.

Another feature of the present invention is the same as any one or moreof the preceding features wherein the inker includes a hollow channelmember directed across the image bearing surface to be developed andsuch channel including an apertured side adjacent the image to bedeveloped for passage of the charged toner particles to the image andwherein at least a portion of the clean-up electrode structure isdisposed at the trailing marginal edge of the apertured channel forcleaning up the background as the recording medium is caused to traversethe channel.

Another feature of the present invention is the same as any one or moreof the preceding features wherein the clean-up electrode structureincludes a grid disposed across the mouth of the apertured side of thehollow inking channel.

Other features and advantages of the present invention will becomeapparent upon a perusal of the following specification taken inconnection with the accompanying drawings wherein:

Brief description of the drawings FIG. 1 is a schematic perspective viewof an electrographic device employing the prior art developer,

FIG. 2 is an enlarged sectional view of the structure of FIG. 1 takenalong line 22 in the direction of the arrow,

FIG. 3 is a view similar to that of FIG. 2 depicting the clean-upelectrode structure of the present invention,

FIG. 4 is an enlarged detail view of a portion of the structure of FIG.3 delineated by line 44, and

FIG. 5 is a view similar to that of FIG. 3 depicting an alternativeembodiment of the present invention.

Description of the preferred embodiments Referring now to FIG. 1, thereis shown one example of a prior art electrographic device 1. In thisexample, an elongated hollow inking channel or developer 2 is employedfor applying liquid toner to charge images 3 deposited upon the chargeretentive surface 4 of electrographic recording paper 5. The chargeimages 3 are deposited by a moving electric stylus 6 carried by aconductive cable 7 and moved back and forth across the recording paper 5by means of a conductive drive pulley 8 and an idler pulley 9. The drivepulley 8 is driven from the output shaft of the reversible rebalancemotor of a self-balancing potentiometric recorder circuit, not shown.The transverse position of the stylus 6 on the recording paper 5 isdetermined by the amplitude of the signal to be recorded by the recorder1.

A negative writing potential as of 500 v. is applied to the stylus 6from a source 11 via the conductive pulley 8 and cable 7. A groundedplate-shaped auxiliary writing electrode 12 is disposed beneath thestylus 6 on the opposite side of the recording paper 5. The potentialdifference across the gap between the end of the stylus 6 and theplate-shaped auxiliary electrode 12 is sufficient to produce aline-shaped charge image pattern upon the charge retentive coating 4 ofthe recording paper 5. The recording paper actually includes aconductive paper backing layer with the dielectric charge retentivelayer formed thereon. In this manner, the very thin, i.e., 4-micronthick, charge retentive layer can be adequately supported.

The recording paper 5 is pulled past the inking channel 2 by means of apair of friction drive wheels comprising an idler wheel 13 and motordriven drive wheel 14. The inking channel 2 comprises a hollow channelmember 15, as of Lucite plastic. The bottom side wall of the channel 15is provided with a longitudinally directed slot 16, as of 0.250 wide,extending nearly the entire length of the channel 2 and nearly acrossthe width of the recording paper 5, as of 4" to 11" wide. An input pipe17 and an output pipe feed a stream of electrographic toner through theinking channel 2 from a reservoir of toner, not shown.

The electrographic toner comprises a colloidal suspension of chargedtoner particles in an insulative liquid vehicle. Although the tonerparticles may have either a positive or a negative charge they arepositively charged in preferred embodiment.

Also, in a preferred embodiment, the toner liquid in the inking channel2 is at less than atmospheric pressure such that the paper 5 is pushedby atmospheric pressure up against the marginal edges of inking slot 16sealing same against escape of liquid toner and assuring that the chargeimage is brought into intimate contact with the toner liquid within thechannel 2, see FIG. 2. As the paper 5 is drawn past the inking slot 16,the positively charged toner particles are attracted out of suspensionand electrostatically bound to the negative charge image 3 to bedeveloped on the charge retentive surface 4 of the recording paper 5.When relatively high concentrations of toner particles are used for highspeed inking or development as, for example, 15% to 30% by weight of theliquid toner, it is found that some of the positive toner particles areleft on the non-charge bearing surfaces, i.e., background areas of theimage to be developed. These particles produce an unwanted backgroundfog.

Referring now to FIG. 3, there is shown a clean-up electrode structure21 for removing the unwanted background fog. The clean-up electrodestructure 21 comprises an array of parallel directed slat-shapedelectrodes 22 which extend lengthwise of and across the inking slot 16from one end to the other. The slat-shaped electrodes 22 areelectrically connected together via lead 23 which is connected to asource of negative voltage 24, as of 250 v., via a switch 25. A segmentof the auxiliary plate-shaped electrode 12' is spring biased, asindicated by the arrow 26, to force the plate into electrical contactwith the conductive backing of the recording paper 5. Under theseconditions, a series of intense electric fields are produced over theimage bearing surface of the recording paper 5 within the inking slot16, as more clearly shown in the detail of FIG. 4. These regions ofintense electric field cause the positively charged toner particleswhich are not electrostatically bound to the charge image to be pulledfrom the background areas of the image and attracted to the clean-upelectrodes 22. The clean-up electrode structure 22 is relatively open.As used herein relatively open means that the electrode structure 22 hasa projected area on the image bearing surface being developed of lessthan 50% of the development area of the slot 16, i.e., the area of theimage hearing surface exposed to the toner. The thin edge portions 27 ofthe clean-up electrodes 22 are preferably slightly recessed, as by forexample, 0.002, from the marginal edges 28 of the slot 16 to prevent theaccumulation of charged toner particles. on the electrodes 22 from beingcarried away by the paper 5 as it is pulled past the electrodes 22.

In a typical example, the slat-shaped clean-up electrodes 22 are 0.010thick. The spacing between adjacent ones of the electrodes 22 should besufiiciently large as of, for example, 0.15" to permit an adequate flowof toner to the image bearing surface 4 of the recording paper 5. As canbe readily appreciated, the trailing edge electrode 22' is the mosteflicient in that with the other electrodes additional background can bedeposited after they have performed their cleaning function.Accordingly, adequate performance is obtainable by use of only thetrailing clean-up electrode 22'. A particularly advantageous electrodearrangement employs only a leading edge electrode 22" and the trailingedge electrode 22'.

In operation, the clean-up potential is preferably applied only duringdevelopment of charge images in order to prevent building up excessiveaccumulations of charged toner particles on the clean-up electrodes 22.Accordingly, the switch 25 may be switched to a grounded position 31during non-development times to prevent such accumulations and to permitpreviously accumulated toner particles to be dispersed from theelectrodes 22. Alternatively, the potential applied to the secondposition of the :switch 25 may be positive to help drive off theaccumulated particles. In a continuously operating developer 2, theswitch 25 may be switched to position 31 periodically. The switchingcycle may be determined by a timer, not shown, or by a device such as acam carried on or geared to the paper pulling friction drive wheels 13and 14.

Referring now to FIG. 5, there is shown an alternative clean-upelectrode structure 32. In this case, the apparatus is essentiallyidentical to that of FIG. 4 except that the clean-up electrodes 22 arereplaced by a grid of conductive wires 33 parallel to the longitudinalaxis of the channel 2. A few support wires may be employed traversingthe slot 16 which wires are preferably not parallel to the direction ofmotion of the recording paper 5. The grid 33 is relatively open and theunwanted background toner particles are pulled from the paper 5 to thegrid which is afiixed over the mouth of the inking slot 16.

Although the inker 2 with the clean-up electrode structure of thepresent invention has been described as employed in conjunction with anelectrographic strip chart recorder 1, it may be employed to advantagein any electrogrophic device, such as for example, a microfilm printer,oscilloscope camera or the like, where it is desired to prevent unwantedbackground fog on the developed images.

Since many changes could be made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In an electrographic apparatus, means for depositing an electriccharge image to be developed on a charge retentive surface of arecording medium, a hollow elongated development channel member directedacross the image bearing surface of the recording medium and having anapertured side adjacent the image bearing surface of the recordingmedium for applying a suspension of charged toner particles in a fluidmedium to the charge image on the recording medium thereby developingsame, said channel member defining a development area wherein the chargeimage is contacted by the fluid toner, said apertured side of saidchannel forming a fluid passageway through which the toner particlesflow from said channel to the charge image to be developed, means forsupplying the fluid toner to the hollow interior of said channel member,a clean-up electrode structure having portions disposed immediatelyadjacent the development area of the image bearing surface of therecording medium, said immediately adjacent clean-up electrode structurehaving a projected area on the development area of less than 50% of the;development area, means for applying an electrical potential to saidclean-up electrode structure of such a magnitude to produce an electricfield for pulling the chargedgtoner particles from thebackground'isurface of the recording medium Where the toner particles'are not electrostatically bound to the charge image to be developed,said clean-up electrode structure includingaa slatshaped electrodeturned on its edge such that the thin marginal edge portion of theelectrode faces the charge image bearing surface of the recording mediumto produce a relatively intense clean-up electric field. the improvementcomprising, means for causing said recording mediu'rn to pass saidapertured side wall of said channel member to define leading andtrailing marginal side edges of said apertured side wall of said channelmember, said side edges of said channel being spaced apart in thedirection of movement of said recording medium past said channel withsaid trailing edge being spaced from a leading edge in the direction ofmovement of said medium, and said clean-up electrode structure includinga slat-shaped clean-up electrode disposed at said trailing marginal edgeof said channel member. v

2. The apparatus of claim 1 wherein said liquid toner is supplied tosaid hollow channel at less than atmospheric pressure, and wherein therecording medium is a web which is pushed by atmospheric pressureagainst the marginal edges of the apertured side wall of said hollowchannel thereby sealing same against escape of liquid toner.

3. The apparatus of claim 1 wherein said clean-up electrode structureincludes a plurality of elongated electrically conductive slat-shapedmembers disposed across the fluid passageway in the apertured side wallof said channel, said elongated .conductive membersj'being directed inthe direction of the longitudinal axis of said channel member and acrosssaid recording web.

4. The apparatus of claim 1 wherein said. recording medium includes aconductive backing portion and a dielectric charge retentive surface,and wherein said means for supplying the liquidsuspension of tonersupplies a positively charged colloidal suspension of such tonerparticles, and wherein said means for applying an electrical potentialto said clean-up electrode structure supplies a potential to a portionof said electrode structure disposed on the developed-side of saidcharge image which is negative relative to the potential applied to theconductive backing portion of said recording medium.

References Cited UNITED STATES PATENTS 2,956,494 10/1960 Tyler et al118-637 3,147,147 9/1964 Carlson 118-637 3,154,546 11/1964 Dirks 118-6373,203,395 8/1965 Liller 118-637 3,299,787 1/ 1967 Kolb et al.

3,342,164 9/1967 Lewis 118-637 3,345,925 10/ 1967 O stensen 118-637 XRPETER FELDMAN, Primary Examiner.

